FIG. 1 schematically represents a known pyrotechnic countermeasure device 3 protecting an aircraft 1, for example, from a threat 2. The threat 2 comes in the form of an infrared homing missile (ADIR).
Device 3 is a pyrotechnic decoy that generates infrared rays when it is fired out of the aircraft 1, when the latter has detected a threat. The infrared rays generated by the decoy 3, which are more intense than the infrared rays generated by the aircraft 1, cause a deviation in the trajectory of the threat. The threat homes in on the decoy 3 rather than on the aircraft 1.
However, these pyrotechnic decoy devices have a number of drawbacks.
To begin with, they are difficult and expensive to design. In addition, they represent a fire risk inside the aircraft if they malfunction. Furthermore, in the event of a false alarm, they seriously compromise the stealth aspect of the aircraft 1. Finally, they are consumable devices that have to be replaced regularly, and are highly specialized to one particular type of threat.
As a consequence, current countermeasure devices are generally of the infrared illumination jammer type.
FIG. 2 schematically represents a known Directional infrared Counter-Measure device (DIRCM).
The aircraft 1 thus includes a missile departure detection device (or Missile Warning System—MWS) in the form of a multiplicity of detectors 6 mounted on the fuselage of the aircraft 1. The detectors 6 detect the launch of a missile 2, track the missile trajectory and identify it as a threat to the aircraft 1.
A device 7 for monitoring the launch detection device transmits the missile trajectory 2 to the control device 8 of a directional countermeasure system.
The control system 8 then triggers the tracking device 4 which tracks the missile and determines its direction in space. The tracking device 4 detects the missile homing system 2 by its laser equivalent surface or “LES”, which is the quantitative value of the “cat's eye” effect. To this end, the tracking device 4 emits a tracking light beam 40 in the direction of the missile homing system 2 and measures the reflected echo in order to measure the LES of the “seek head”.
The control device 8 also controls a jamming light beam 50 that has an angular opening (γ) produced by an infrared light source 5. The light source 5 generally uses discharge-type infrared lamps, whose spectrum covers a broad spectrum of wavelengths, from the visible up to the far infrared. Once tracking has locked on correctly and the missile is securely within the beam of the light source 5, the infrared illumination 5 is transmitted toward the missile homing system 2 in a specific sequence in order to cause jamming of the missile 2, so that it no longer represents a threat to the aircraft 1.
These discharge light sources 5 also have many drawbacks however.
Due to the coverage required from the light source, beam concentration requires a voluminous optical device (large aperture, long focal length, etc.) which is very bulky and heavy. The jammers based on discharge lamps are therefore difficult to accommodate on board aircraft. In addition, the illumination power available is rather limited, thus greatly limiting the effectiveness of such jammers using discharge lamps.
Thus, with the evolution of the technology, it is now possible to create infrared laser sources that emit wavelengths of between 3 and 5 micrometers. This evolution of the technology allows us to achieve a significant improvement in tracking and countermeasure devices. In fact, with a laser source, it is possible to considerably increase the beam strength 50 and/or 40 with a far smaller source than the discharge type, and much closer to the diffraction limit. Because the laser source is a coherent source, all the beam energy is concentrated in a single wavelength.
Increasing the beam strength over one or more specified wavelengths has certain advantages.
A laser beam allows us to deposit a higher light level than that of the discharge lamps at the aperture entry of the homing device of the threat.
The laser energy can be contained in several rays (typically two or three) so as to be able to effectively illuminate all types of missile homing device.
Because of its high brightness (the beam is close to diffraction) a laser beam can be collimated with an optical device of small dimensions, rendering the latter easy to fit into aircraft while also providing a range of performance that is acceptable for the required functions of active tracking, of identification AD and jamming DIRCM.
A laser beam can be reflected to a greater extent by the “seek head”, so that missile identification 2—in particular by the modulation that it imparts to the reflected laser beam—is facilitated. A quality identification of the threat 2 allows us to transmit an effective jamming code, meaning a code that is truly designed for the missile type 2. The source is very directional, thus enhancing the general stealth aspect of the aircraft.
However, tracking and countermeasure devices using a laser source present a number of drawbacks.
Because of the narrowness of the laser beam produced by the laser source (in general less than 1 milliradian), the tracking device 4 must be capable of very precise tracking of the missile infrared detector 2.
In addition, the coherence characteristic of the laser sources imposes the use of a beam shaping, orientation and stabilization device which generates no interference that could have negative consequences for general system operation and particularly on the effective jamming of the missile homing system.
It is therefore very difficult to design a tracking and countermeasure device that can rapidly and simply track the missile where an infrared laser source is used.